1. Field of the Invention
The present invention relates to a wireless or radio telephone system which realizes a wireless communication with a party at the end of a wire telephone line through a base unit connected to the wire telephone line.
2. Description of the Prior Art
In general, the radio telephone system of the type referred to has such an arrangement as shown in FIG. 1. In the drawing, reference numeral 1 denotes a base unit, 2 a wireless or radio telephone set, 3 a wire telephone line. The base unit 1 electrically connected to the wire telephone line 3 is coupled to the radio telephone set 2 through a radio communication line.
More specifically, a signal coming from the wire telephone line 3 is sent through a turned-on line relay 27 (which will be explained later) and a hybrid circuit 4 to a transmitter 5 as its modulation input to be modulated therein. The signal modulated in the transmitter 5 is sent to an antenna duplexer 6 and then radiated from an antenna 7 to the radio telephone set 2 in the form of electro-magnetic (radio) waves.
Electro-magnetic waves radiated from the radio telephone set 2, on the other hand, are received by the antenna 7 and then sent through the antenna duplexer 6 to a receiver 8 to form a demodulated signal therein. The demodulated signal is sent from the receiver 8 through the hybrid circuit 4 to the wire telephone line 3.
A synthesizer 9 provided in the base unit 1 functions to provide a signal of a frequency corresponding to a radio channel (in the illustrated example, the radio telephone communication system being assumed to have two types of channels, i.e., speech and control channels) to the transmitter 5 and the receiver 8 respectively.
On the basis of such a channel frequency signal (which specifies a radio channel), the transmitter 5 modulates its incoming signal from the telephone line 3 while the receiver 8 demodulates waves received at the antenna 7, respectively. In particular, the demodulated output of the receiver 8 is partly inputted to a received-field strength detector 10 to be used for judgement of the intensity of electric field in the radio communication. This detector 10 is generally called the carrier or noise squelch circuit. Another part of the demodulated output of the receiver 8, in particular, when the above control channel is specified, is inputted, as a data signal contained in the received and demodulated waves, to an identification signal detector 11 for collating an identification signal determined by the base unit and the radio telephone set 2 and also to a control circuit 12. This identification signal is usually referred to as the ID code.
Also inputted to the control circuit 12 are outputs of the detectors 10 and 11 which outputs are used, together with the demodulated output of the above data signal, to control an interconnection (which will be detailed later) between the base unit 1 and the radio telephone set 2. Further, this control circuit 12 controls the synthesizer 9 to switch the radio channels, and also sends a transmission data signal to the transmitter 5 as its modulation input.
The radio telephone set 2, like the base unit 1, contains an antenna 13, a receiver 14, a transmitter 17, an antenna duplexer 18, a synthesizer 19, a received-field strength detector 20, an identification signal detector circuit 21 and a control circuit 22.
A demodulated electric output of the receiver 14 is applied to a telephone handset 15 and is converted into a voice signal. A telephone mouthpiece 16 converts user's voice signal to an electric signal and applies it to a transmitter 17 as its modulation input. The transmitter 17 sends the modulated signal to the antenna duplexer 18 and then the antenna 13 to be radiated from the antenna in the form of electro-magnetic waves.
The synthesizer 19, the received-field strength detector 20 and the identification signal detector 21 are substantially the same as the equivalents in the base unit 1, and the control circuit 22 performs overall control over the radio telephone set 2. A loud speaker 23 comprises a sounder which emits a ringing tone in an incoming call mode.
The control of the radio telephone system at the time of receiving an incoming call is schematically as shown by a flowchart in FIG. 2.
More in detail, when the base unit 1 in a wait mode detects at a ringing-tone detecting circuit 24 a ringing tone (having a frequency of 16 Hz) received from the wire telephone line 3 (step 101), the control circuit 12 examines whether or not the associated control channel (C-CH) is idle (step 102). If it is idle, then the control circuit controls the synthesizer 9 to cause the oscillation frequency of the synthesizer to be set at the frequency of the control channel so that the transmitter 5 is turned on to transmit the received signal (step 103). This received signal contains a speech-channel (S-CH) specifying signal.
On the side of the radio telephone set 2 in its wait mode, on the other hand, the synthesizer 19 is kept ON during a constant time t1 set by a timer circuit (incorporated in the control circuit 22) to have an oscillation frequency set at the frequency of the control channel, and the receiver 14 is also turned on (step 104). At this time, if the telephone set 2 receives an incoming signal (step 105), then the transmitter 17 is turned on (step 106) to send an answer signal in response to the incoming signal (step 107), whereby switching is made to the specified speech channel (step 113). If the telephone set receives no incoming signal, then the synthesizer 19 and the receiver 14 is kept OFF during a constant time t2 (step 108). The intermittent signal-receiving operation caused by the turning on and off of the receiver 14 is known as a battery saving phenomenon.
The base unit 1, when detecting radio waves radiated from the radio telephone set 2 at the received-field strength detector 10 (step 109), stops the transmission of the incoming signal (step 110). At this stage, when there is no detection of radio waves from the telephone set causes the base unit to repeats the transmission of the incoming signal up to a predetermined number of times n (step 111). The reason for this is that the radio telephone set 2 is arranged to intermittently receive the signal and cannot receive any signal during the aforementioned time t2. Further, the reason why the transmission is terminated after the n transmissions is to avoid the useless occupation of the control channel, for example when the power supply of the telephone set 2 is turned off or when the telepone set is located too far from the base unit.
Next, when the base unit detects an identity of the ID code (step 112), it switches the current speech channel to a speech channel (S-CH) specified by the received signal. If the control circuit fails to detect the ID code identity, then the ID code corresponds to a response from the other radio telephone sets and thus the base unit waits for the extinction of the call from the wire telephone line 3 and then returns to its wait mode (step 125).
The base unit 1, after having been switched to the speech channel, then transmits a bell sounding signal (step 115). The radio telephone set 2, when receiving the bell sounding signal (step 116), sends it to the sounder 23 to sound a ringing tone (step 117). In response to sounding of the ringing tone, when the telephone set user turns on a switch 25 to put the telephone set in its off-hook state (step 118), the telephone set 2 transmits an off-hook signal (step 119), whereby the telephone set 2 is put in its communication mode (step 123).
On the other hand, the base unit 1, when receiving the off-hook signal (step 120), stops the transmission of the previous bell sounding signal (step 121) and closes the line relay 27 to establish a speech loop with the wire telephone line 3, which also puts the base unit 1 in its speech mode (step 122).
When there is no detection of radio waves from the telephone set 2 and when the number of transmissions reaches n, the base unit detects the completion of signal reception from the wire telephone line (step 124) and returns to its wait mode, thus preventing the unnecessary repetition of the incoming signal operation.
FIG. 3 shows a flowchart for explaining the operation of the system when signal transmission is done from the side of the radio telephone set 2.
More specifically, when the user turns on the switch 25 to call a party at the end of the wire telephone line 3, the control circuit 22 makes the determination to shift the telephone set to its signal transmission mode on the basis of the switch 25 turned on (step 201), and controls the synthesizer 19 to cause the oscillation frequency of the synthesizer 19 to be locked at the control channel and to cause the receiver 14 to be turned on to thereby receive radio waves on the control channel (C-CH) (step 202). The intensity of the electric field of the received waves on the control channel is detected by the received-field strength detector 20. If the field intensity of the received signal does not exceed a predetermined level, then the control circuit determines that the control channel is idle (step 203) and turns on the transmitter 17 to transmit an ID code signal inherently assigned to the own telephone set (step 204). The base unit 1, when receiving this ID code signal, judges whether or not the received ID code coincides with an ID code uniquely allocated to a combination of the radio telephone set 2 and the base unit 1 (steps 205 and 206). If the base unit determines a coincidence, then it turns on the transmitter 5 to transmit an answer signal (containing the ID code and speech channel (S-CH) specifying information) for the telephone set 2 (step 207).
The radio telephone set 2 receives such an answer signal from the base unit 1 (step 208) and determines whether or not the ID code contained in the answer signal coincides with its own ID code (step 209). An ID code coincidence causes the oscillation frequency of the synthesizer 19 to be swiched to the frequency of the speech channel specified by the base unit 1 (step 210). The base unit itself, after transmitting the answer signal, also changes the oscillation frequency of the synthesizer 9 to the frequency of the speech channel specified for the purpose of the speech (step 211). As a result, the base unit 1 is connected to the radio telephone set 2 through a speech channel specified by the base unit 1. Thereafter, the user can call the party telephone set connected to the wire telephone line 3 by means of the dialling operation of dial keys 26, thus enabling the conversation with the party.
The ID code signal is transmitted from the radio telephone set 2 to the base unit 1. However, in the event where the telephone set 2 cannot receive an answer signal from the base unit 1 even after passage of a time t3 for such reasons as a distance that is too far between the base unit 1 and the telephone set 2 or the like, the control circuit 22 controls the loud speaker 23 to cause an alarm tone to be generated from the speaker 23 at the stage of passage of the time t3, thus informing the user of the disabled interconnection between the telephone set and the base unit (steps 212 and 213).
Although the speech communications or the interconnection control communications prior to the speech communications between the base unit 1 and the radio telephone set 2 has been explained in connection with the case where such communications is carried out through such individual radio channels as the aforementioned speech channel (S-CH) and the control channel (C-CH) in the illustrated embodiment, there may also be provided such a radio telephone system that only one sort of radio channel (speech channels) are used to carry out these interconnection control and speech communications.
In the both types of radio telephone systems, a predetermined threshold value (for example, 20 dB.mu.V) indicative of a predetermined intensity of the electric field of the received signal is previously set in the received-field stregth detector 20 so that, when the intensity of the electric field of the received signal shown by the output of the receiver 14 exceeds the threshold value (20 dB.mu.V), the detector 20 judges that the intensity of the received electric field is at such a level that allows radio communications. If the radio telephone set 2 kept in its communication mode is moved far from the base unit 1 and the intensity of the received electric field becomes correspondingly much smaller than the threshold value (20 dB.mu.V), then the received-field strength detector circuit 20 determines that the field intensity of the received-signal does not reach such a level that allows radio communications and applies a signal indicative of this effect to the control circuit 22.
When the control circuit 22 receives this signal indictive of the lack of the received electric field intensity, it causes the loud speaker 23 to generate such an out-of-communication-range signal as an alarm sound. This allows the user to know the reduced electric field intensity. However, if such a reduced field intensity continues, that is, if a predetermined time (for example, 10 seconds) elapses after the signal indicative of the lack of the electric field intensity has generated, then the control circuit 22 disconnects the once-connected radio line. In other words, the control circuit 22 reports the effect to the base unit 1 via the transmitter and thereafter, turns off the receiver 14 and the transmitter 17 to return the radio telephone set to its initial operational state (the state of the first step in FIGS. 2 and 3). The base unit 1 itself is returned to the initial state in response to the aforementioned report from the radio telephone set 2.
This arrangement is intended to secure the communication quality. That is, when the intensity of the received electric field is reduced to a low level, it becomes difficult to secure the articulation of the speech due to the noise of the radio line. This arrangement is further intended to prevent any interference with other radio telephone sets and secure a high reliability in transmission of the dial signal (which correspond to the processings of the steps 212 and 213 in FIG. 3).
In this way, the prior art radio telephone system, which is arranged to allow the speech maintenance and the like only when the received electric field intensity is kept at a level exceeding the constant threshold value (for example, 20 dB.mu.V) preset for the received electric field intensity, is surely effective in securing the aforementioned speech quality, preventing any interference with other radio telephone sets or securing the reliability of the dialling signal; but is ineffective in satisfying such a demand, for example, that "the user wishes to move the telephone set to a place (location) out of the communication range where the intensity of the received electric field is kept to be higher than the aforementioned threshold value, while securing the speech communication". That is, the prior art is defective from the viewpoint of a so-called flexibility.